In an automatic transmission control valves supply hydraulic pressure to clutches and bands to engage each gear. A pump draws fluid from a sump in the bottom of the transmission and feeds it to the hydraulic system. Once the clutch is no longer required to be applied, the fluid is drained back to the sump. When the transmission is in park, neutral, or the engine is turned off, the pressurized fluid can be completely drained from the clutch and collected in a sump.
During this time, rotating clutches in a clutch-to-clutch transmission experience drain down issues that cause air to get into the clutch. Despite a bleed orifice, air becomes entrapped in the clutch. The trapped air causes the first shifts after complete drain down to flare. In order to eliminate air entrapped in the clutch it is desirable to purge air from the clutch of the transmission.